>If you'll need to withdraw the principal before retirement: Roth (you can withdraw principal penalty free from a RothIRA).

I'm not the original commenter but in regards to the advantages and situations mentioned, they apply to both Roth IRAs and Roth 401k's.

Although, check your company's plans as to when you can take early distributions. If you can't unless you retire or leave the company then "If you'll need to withdraw the principal before retirement: Roth (you can withdraw principal penalty free from a RothIRA)." might be moot. I know with my companies plan I don't believe I can even withdraw my Roth 401k contributions without quitting or retiring. With most Roth plans you are allowed to withdraw your contributions tax and penalty free, but you won't be able to "re-add" above and beyond the yearly annual contribution limits, which is why you shouldn't withdraw them.